Conventional bicycle frames use tubes joined into closed frameworks to interconnect the components (front fork/handlebar assembly, seat, crankset, rear wheel) and transmit the loads therebetween. The tubes also keep the components in spacial relation to one another, i.e., they maintain the components in relative proximity.
The traditional bicycle frame includes a top tube, a relatively short head tube, a seat tube, a down tube, a bottom bracket, a pair of chain stay tubes, a pair of dropouts, and a pair of seat stays. The top tube connects at its front end to the top of the head tube, and extends backward to the top of the seat tube. The down tube connects at the front end to the bottom of the head tube and extends downwardly and rearwardly to the bottom bracket where the seat tube and the down tube are connected. The pair of chain stay tubes extend rearwardly from the bottom of the seat tube to the dropouts. The chain stay tubes are generally connected to a bottom bracket at the base of the seat tube. The pair of seat stays connect directly or indirectly to the top of the seat tube. The seat stays extend backward from the seat tube to the dropouts. The seat stays generally include two seat stay tubes that converge from the dropouts toward the seat tube. The dropouts support the rear wheel axle. This conventional frame design is called the “diamond” frame, because when viewed from the side, the top tube, down tube, chain stay tubes, and seat stays enclose a diamond-shaped space.
The rear portion of conventional rigid frames are made up of a pair of generally horizontal chain stay tubes that are welded at their front ends to the bottom bracket and are adapted to receive the rear wheel axle near their rear ends. A pair of seat stays are welded at their upper ends near the top of the seat tube and are welded near the ends of the chain stay tubes at their lower ends. The seat tube, chain stay tubes, and seat stays form a rigid generally triangular configuration, with the rear wheel carried at the rear apex.
Such frames are used for both road bikes and mountain bikes. The terrain on which a bike is used dictates the load requirements for the bike and the performance characteristics of the components. For example, since mountain bikes are used on rough terrain and often are ridden over roots and jumps, they must be more durable than road bikes, which encounter relatively smooth surfaces. Further, the conventional rigid arrangement is unsatisfactory for comfort and optimum performance on rough terrain. Thus, provision of a pivoting spring suspension for the rear wheel is desirable on a bicycle to be used on rough terrain, such as mountain bikes, cross-country bikes or trail bikes.
Some prior art bicycles have rear shock absorbing assemblies in which the rear axle pivots about a single elevated pivot point when subjected to a shock force. This generally results in the rear wheel axle moving upwardly in an arc rather than moving vertically upward in a substantially linear fashion.
To accommodate a rear suspension, some bicycles have a rear suspension linkage system, including an upper link to which the seat stays are pivotally attached at an upper end adjacent the seat tube, while the chain stay tubes are pivotally attached to the bottom bracket at a forward end and a rear pivot axis adjacent the hub of the rear wheel at a back end. The lower end of the seat stays are pivotally attached to the back end of the chain stay tubes. Typically, a shock absorber is located at the upper end of the seat stay, such that the shock absorber is rigidly connected to the seat stay at one end with the other end being pivotally attached to the seat tube. These linkage systems have several drawbacks in that they are mechanically complicated, require a large number of parts, and add unwanted weight. Therefore, a need exists for a rear suspension that uses less complicated and light weight linkage systems.
Other rear suspensions for “soft-tail” designs of bicycles use the flex of chain stay tubes for suspension; however, they bend along the length of the chain stay tubes. When chain stay tubes flex over their entire length, it is difficult to dictate the performance characteristics of the rear suspension. This can be accomplished, however, by isolating the specific location where the chain stay tubes bend. Therefore, a need exists for a chain stay tube that pivots around a specific point.